1. Field of the Invention
The invention concerns a motor brake device for a turbocharged internal combustion engine, a process for operating the motor brake device as well as an internal combustion engine with such a motor brake device.
2. Description of the Related Art
In the area of turbocharged internal combustion engines it is known that in addition to the motor brake of the internal combustion engine the turbocharger may also be provided with its own brake assist device. In a typical manner of operation such a brake assist device always brings about a transformation of the turbocharger from a power output or drive device to a brake device when a brake assist process is necessary. This generally occurs by modification of the exhaust and/or suction side control of the turbocharger in such a manner that a movement of the cylinder pistons is changed from a low as possible power output loss to a large as possible power output loss and namely then when the braking of the turbocharger occurs. Such an absorption of power is based on the principle of an air compressor, that is, the cylinder pistons perform work on the air contained within the cylinders of the motor when braking is necessary.
Here there exist two fundamentally different concepts by which a brake assist for the internal combustion engine can be realized with the aid of the turbocharger:
According to a first process it is attempted, by reducing the effective cross-section of the exhaust gas side, to increase the pressure in the exhaust pipe, in order to thereby create a so-called counter pressure or back pressure, which is transmitted to the gas in the cylinder volume of the internal combustion engine. On the basis of the higher pressure in the cylinder internal space the cylinder pistons must perform a greater amount of work, which leads to braking. In practice this is carried about in various ways:
In DE 195 43 190 A1 a brake assist device for a charged internal combustion engine is described, which includes an exhaust gas turbocharger with variably adjustable turbine geometry via adjustable guide vanes. The channeling means include guide vanes, which can be so adjusted with the aid of an adjusting mechanism, that the effective, that is, the useful turbine cross-section of the turbine geometry, is changed. Thereby, depending upon operating condition of the internal combustion engine, various elevated exhaust gas pressures can be realized in the segment between the cylinders and the turbocharger, whereby the output of the turbine and the output of the compressor can be adjusted according to need.
In order to cause a motor braking during operation of the internal combustion engine, this array of guide vanes is brought into a constriction orientation in such a manner that the effective turbine cross-section is significantly reduced. In a guide vane assembly segment between the cylinders and the turbine a high exhaust gas pressure builds up with the consequence that exhaust gas flows with high velocity through the channels between the guide vanes of the turbine and impact the turbine wheel with a high impulse. The turbine output is transmitted to the compressor, whereupon the charge air supply to the motor is placed under elevated charge pressure by the compressor. Thereby the cylinder on the charge air side is acted upon with increased charge air, at the exhaust gas side there is an elevated exhaust gas pressure between the cylinder outlet and the turbocharger, which counter acts the output of the air compressed in the cylinders by opened brake valves in the exhaust pipe segment. In the motor braking operation the piston must perform increased compression work against the high overpressure in the exhaust line, whereby depending upon the positioning of the guide vanes a more or less strong braking effect is achieved.
Supplementally or alternatively thereto turbines provided with guide vanes can also have a flap valve, which is provided in the exhaust gas line downstream of the turbine. This flap can be pivoted perpendicular or substantially perpendicular to the exhaust gas line in the braking operation of the motor brake device and reduces thereby the effective cross-section in the exhaust gas line, whereby upstream in the direction of the cylinder outlet the pressure in the exhaust gas line increases and therewith a braking effect is achieved. A turbocharger with such a flap is described for example in DE 40 24 572.
According to a second concept an exhaust gas return line can be provided for elevating the motor brake effectiveness, which is activated during the motor braking operation. Therein exhaust gas out of the exhaust pipe which during the motor braking operation includes various amounts of uncombusted exhaust gas air and which due to compression in the cylinders has an elevated temperature level, is supplied again to the cylinders of the internal combustion engine.
In DE 198 53 127 A1 such a motor brake device with exhaust gas recirculation is described. There the exhaust gas is branched off prior to the turbocharger, is directed in the direction of the cylinder inlets, and is mixed together with the combustion air compressed in the compressor of the turbocharger and supplied to the cylinders. A back-flow valve is provided in the line for exhaust gas recirculation. This back flow valve is necessary here in order to balance out pressure differentials between the exhaust gas line and the charge air line.
All of the above mentioned brake assist devices of a turbocharged internal combustion engine are however designed only for so-called single stage turbochargers. Modern turbochargers can however have a two stage charge system.
An internal combustion engine equipped with such a two-stage charge system is described for example in German patent publication DE 198 37 978 A1 and DE 195 14 572 A1. In such a two-stage charged internal combustion engine the set of turbochargers respectively includes one high-pressure stage and one low-pressure stage arranged in sequence to each other. The exhaust gas leaving the motor first flows through the high-pressure turbine and subsequently the low-pressure turbine. In the same manner the charge air for supplying the cylinders is first compressed by a low-pressure compressor and subsequently by a high-pressure compressor and supplied to the charge air side of the internal combustion engine, in certain cases following cooling of the charge air in a heat exchanger. In a typical mode of operation the turbocharger during low RPM of the internal combustion engine is operated in two stages. As the RPM increases operation can be switched to the single low pressure compressor, wherein for example by means of an exhaust gas sided bypass line the high pressure turbine can be completely or at least partially bridged over or bypassed. In this case the high-pressure compressor could also be completely bypassed via a pipe switch or valve provided in the charge air side.
In such a two-stage turbocharger the turbines and compressors respectively arranged in series are designed for different charge pressures. In practice this has the consequence, that very large constructive expenditure is necessary for realizing the above-mentioned brake assist device. In order for example to obtain one optimal brake assist module for each of the respective different modes of operation of the two-stage turbocharger, a large number of pipe switches indispensable in order respectively to achieve the desired pressures in the exhaust pipes and charge air lines. Such brake devices are thus very expensive in their manufacture, wherein however the increased expenditure does not bring about an improvement in braking quality. In particular in the case of very small turbochargers, which are employed primarily in internal combustion engines with very small engine compartments, such a brake assist device has not been provided in satisfactory manner until now.